1. Field of the Invention
The present invention relates to a method of controlling the mode of driving electric vehicles that run along a track, as electric locomotives and electric railroad cars, so as to accelerate the electric vehicle at a maximum acceleration and to decelerate the same at a maximum deceleration by enhancing the adhesion acting between the wheels of the electric vehicle and the rail.
2. Description of the Prior Art
As is generally known, a railroad vehicle, as an electric railroad vehicle, runs as the wheels thereof are rolled along rails 1 as illustrated in FIG. 1. The mode of rolling of the wheel is dependent on the correlation between the ratio of an adhesion F that is applied to the wheel 2 by the upper surface of the rail 1 to a weight W applied to the upper surface of the rail 1 by the wheel 2, namely, coefficient of adhesion .mu.=F/W, and the ratio of the difference of the circumferential speed V.sub.s of the wheel from the speed V.sub.p of the center P of the wheel 2 relative to the rail 1 to the speed V.sub.p of the center P of the wheel 2 relative to the rail 1, namely, slip factor .tau.=(V.sub.s -V.sub.p)/V.sub.p. The relation between the coefficient of adhesion .mu. and the slip factor .tau. is represented by a curve a in FIG. 2, in which the coefficient of adhesion .mu. and the slip factor .tau. are measured on the axis of ordinate and on the axis of abscissa, respectively.
A conventional controller for controlling the mode of driving an electric vehicle controls the mode of driving an electric vehicle on the basis of such a relation between the coefficient of adhesion .mu. and the slip factor .tau.. As illustrated in FIG. 3, the conventional controller comprises wheels 2a, 2b, 2c and 2d supported on four axles, respectively, so as to roll along a rail 1, revolving speed detectors 11, 22, 33, 44 for detecting the respective revolving speeds V.sub.s1, V.sub.s2, V.sub.s3 and V.sub.s4 of the wheels 2a, 2b, 2c and 2d, a relative speed detector 5, as a Doppler radar, for detecting the speed V.sub.p of a vehicle body 7 relative to the rail 1, and a propulsion/ braking controller 6 which calculates the slip factor .tau. on the basis of the speed V.sub.p given thereto from the relative speed detector 5 and the revolving speeds V.sub.s1, V.sub.s2, V.sub.s3 and V.sub.s4 given thereto from the revolving speed detectors 11, 22, 33 and 44 and controls the revolving speed V.sub.s1, V.sub.s2, V.sub.s3 and V.sub.s4 of the wheels on the basis of the calculated result. A detector of the radar system using the Doppler effect of radio waves or light is employed as the relative speed detector 5. In order to meet a required measuring accuracy, the radar uses radio waves of a frequency in the tens GHz band or light. The controller 6 calculates .tau.=(V.sub.s -V.sub.p)/V.sub.p by using the speed V.sub.p given thereto from the relative speed detector 5, and the revolving speeds V.sub.s1, V.sub.s2, V.sub.s3 and V.sub.s4 of the wheels 2a, 2b, 2c and 2d given thereto from the revolving speed detectors 11, 22, 33 and 44, and then the controller 6 controls the revolving speeds V.sub.s1, V.sub.s2, V.sub.s3 and V.sub.s4 of the wheels 2a, 2b, 2c and 2d with reference to the curve a of FIG. 2 so that a maximum coefficient of friction .mu.max is obtained.
The conventional method of controlling the mode of driving electric vehicles has the following problems. Since a detector of the radar system using a radio wave or light is employed for detecting the speed of the vehicle body 7 relative to the rail 1 and the frequency band of the radio wave or light applicable to the detection is limited to a very narrow range, the detector for measuring the speed V.sub.p is very expensive and only limited special manufacturers are capable of the manufacture and maintenance of such a detector. When a radio wave is used, the operation of the detector is disturbed by induction troubles attributable to the neighboring communication systems. On the other hand, when light is used, the detector is bothered by the soiling of the components of the optical system such as a lens and the wear of the lamp. Furthermore, since the detector of either the radio system or the optical system uses the reflected wave (reflected light) for measurement, the measuring accuracy is deteriorated when the surface of the rail 1 is very flat, when there are snowdrifts or water puddles are in the track or when there are spaces between the crossties as in sections of the track on bridges, and, in the worst case, the measurement of the running speed of the electric vehicle is impossible.
A method of controlling the minute idle rotation of the wheels by detecting the slip of wheels from the data of the individual axles is proposed in a paragraph "Principles of Operation" of "Faively Anti-skid AEF 83 Braking Under Control", which describes that "The reference speed calculated by the Faively anti-skid device is very close to actual vehicle speed. This reference speed is the result of averaging the reconstituted speed of four axles by very elaborate means." However, this method is incapable of highly accurate control of the mode of driving the vehicle, because the control operation of this method is based on the average value of the variation of the revolving speeds of the four axles.